Techniques are known for controlling emissions of sulfur dioxide (SO.sub.2) or nitrogen oxides (NO.sub.x) which are toxic oxidation products emitted from combustion systems such as power plants.
For SO.sub.2 control, wet scrubbing towers permit effluent gases to pass through beds of limestone, dolomite, and other calcium-containing compounds or catalysts. U.S. Pat. Nos. 3,962,864; 4,178,357; 4,302,425; 4,304,550; 4,313,742; and 4,562,053 illustrate various devices for cleaning flue gases. Wet scrubbing devices tend to be expensive because their complexity requires high operating costs. They also suffer from severe corrosion and plugging problems.
The wet scrubbing process has constantly been modified in the attempt to down-size the scale of equipment. U.S. Pat. No. 4,861,577 describes a method wherein exhaust gas is absorbed into a scrubbing solution which is then treated in an autoclave to decompose such compounds as thiosulfate and polythionates into elemental sulfur and sulfate. U.S. Pat. No. 5,019,361 discloses an amine salt absorbent that purportedly permits high recovery of sulfur dioxide with smaller equipment.
A technique related to wet scrubbing is the spraying of water slurries or dry powders of sulfur sorbents. Various spraying devices, some of which are used in conjunction with wet scrubbers, are shown in U.S. Pat. Nos. 4,001,384; 4,323,371; 4,419,331; and 4,530,822. These devices facilitate gas/liquid contact by atomization of liquids into flue or stack gases. Injection of sorbents can be implemented in the furnace or post-furnace zone, depending upon the thermodynamic and kinetic processes involved, thereby increasing the flexibility of the spraying technique.
Sulfur sorbent particles, ideally, should be small in size, porous, and able to mix well with the gases that are to be cleaned of pollutants. Typical sulfur sorbents are listed below ("Alternative SO.sub.2 Sorbents," PSI Technology Company Report PSI-538/TR-744, 1987. RESEARCH PARK. P.O. Box 3100, Andover, Mass. 01810):
______________________________________ Sorbent Class Sorbent Type Formula ______________________________________ Lime/Limestone Hydrated Dolomite Ca(OH).sub.2.Mg(OH).sub.2 Hydrated Lime Ca(OH).sub.2 Limestone CaCO.sub.3 Dolomite CaCO.sub.3.MgCO.sub.3 Alkali Trona Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O Nahcolite NaHCO.sub.3 Mixed Cation Shortite Na.sub.2 Ca.sub.2 (CO.sub.3).sub.3 Gaylussite Na.sub.2 Ca(CO.sub.3).sub.2.5H.sub.2 O Pirssonite Na.sub.2 Ca(CO.sub.3).sub.2 H.sub.2 O Eitelite Na.sub.2 Mg(CO.sub.3).sub.2 ______________________________________
Upon injection into high-temperature environments, sulfur sorbents containing calcium undergo calcination or decomposition to an oxide (CaO). The same holds true for magnesium-based sorbents which oxidize to MgO, as taught in U.S. Pat. No. 4,874,591. The internal surface area and porosity of sorbents increase drastically during calcination. However, at higher temperatures, above 1000.degree. C. for example, sintering occurs progressively, and the calcium oxide particles rapidly lose porosity and internal surface area.
Sulfation occurs subsequently to calcination. In other words, CaO reacts with SO.sub.2 and H.sub.2 S gases to form solid sulfate, sulfite or sulfide (CaSO.sub.4, CaSO.sub.3, or CaS). The extent of magnesium oxide reaction with SO.sub.2 is not defined but is known to be much smaller than with calcium oxide. However, dolomite-based sorbents result in higher SO.sub.2 capture efficiency than calcite-based sorbents which might be due to the larger total surface area of the former. (Cole et al., Paper 16 Proceedings: 1986 Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies. 1, EPRI CS-4966. December 1986). Furthermore, it has been reported that the presence of MgO promotes the catalytic oxidation of any existing SO.sub.3 to SO.sub.2. (Flagan et al., Fundamentals of Air-Pollution Engineering. Prentice-Hall, New Jersey, 1988.) The reactions may potentially occur in the internal pore surface of the CaO particles as well as upon the external particle surface. However, because of the high molar volume of the calcium sulfate (3.3 times that of CaO) the reaction product induces pore filling and entrance closure in the sorbent particle. Hence, the outer layer reacts first to form calcium sulfate, the pores plug up, and the core remains unreacted. Although the sorbent particles may be ground to micron size to minimize this waste, such an adjustment step is prohibitively expensive for power plant applications and other large-scale uses.
NO.sub.x can be controlled by either minimizing its formation during the combustion processes or destroying it (after it forms) in the effluent of combustion systems. Control of NO.sub.x formation in combustion systems can be achieved by modifying the design and the operating conditions of the furnaces so that the fuel burns in separate fuel-lean and fuel-rich stages.
A number of names have been applied to the various implementation of staged combustion, including: overfire air, off-stoichiometric combustion, and low-NO.sub.x burners. In staged combustion only part of the air required for complete combustion is supplied with the fuel. The remaining air is supplied through separate "overfire ports." This process provides the time and conditions required for NO.sub.x reduction to N.sub.2. If this process is carried too far, HCN will be formed in large quantities (Flagan et al., supra). "Low-NO.sub.x " burners utilize burner aerodynamics to slow the rate at which fuel and air are mixed (U.S. Pat. No. 4,381,718). The degree of control that can be reached by this method is limited by the need to achieve complete combustion within the volume of the combustor.
Post-combustion destruction of NO.sub.x includes techniques such as selective reduction of NO by ammonia (NH.sub.3), urea (H.sub.2 NCONH.sub.2), or isocyanic acid as well as selective catalytic reduction techniques (Flagan et al., supra).